1. Field of the Invention
The present invention relates to a technique for digital split-channel transmission and, more particularly, to a technique using interpolative coders which convert samples of an analog signal into unique digital number pairs for separate transmission to a remote receiver for inverse decoding. Such technique permits either half of the transmitted signal to be used to reproduce to a fair degree of accuracy, and both halves of the transmitted signal to provide high-quality reproduction of, the original analog signal.
2. Description of the Prior Art
Reliability in the transmission and reception of voice signals over signal paths which are subject to frequent outages, as might be found, for example, in mobile telephony, is important. Prior art techniques for increasing reliability of the recovered signal include the use of error correcting codes and multiple transmission of the signal.
Another technique is disclosed in U.S. Pat. No. 4,229,822 issued to S. M. Bench on Oct. 21, 1980. There, information is encoded into digital messages having a start code followed by one or more data blocks. The start code identifies the beginning of the data block that follows and enables synchronization of clock circuitry to the received data frequency. The data blocks have N digital words with M binary bits where one word is a parity word and N-1 words are data words. Each of the data words has a data portion and parity portion coded for correction of at least one error. Reliability is enhanced by a data detector which discriminates between data and noise or voice to provide an indication of the presence of data. In transmitting the digital messages, the bits of the N words in each data block are interleaved to provide protection against error bursts.
Another technique is disclosed in the article "The Design of A 16 Kb/s Split-Band Adaptive Predictive Coder For Noisy Channels" by R. S. Cheung et al. in ICASSP 81 Proceedings, Altanta, Ga. Mar. 30-Apr. 1, 1981, at pages 631-635. There, the frequency contents of an input signal are evenly split into two subbands via quadrature mirror filters and, after sampling at the Nyquist rate, adaptive predictive coding (APC) is applied to the subband waveform samples. A bit allocation scheme is used to dynamically alter the quantizer bit assignment according to the energies of the two subbands. At the receiver the digital subband signals are decoded using APC and the difference between the two decoded signals forms an estimate of the original signal.
The problem remaining in the prior art is to provide a technique which is easy to implement which can provide high quality reproduction of the original signal with all parts of the signal being received and a fair degree of accuracy when only a part of the signal is received.